The present invention relates to improvements in the field of laser imaging. More particularly, the invention is concerned with an improved laser scanning system for use in laser imaging.
Laser imaging apparatuses are well known in the art. These apparatuses are capable of producing on a photosensitive material two-dimensional images having resolutions of up to about 8,000 dots per inch. One type of such apparatuses generally includes a laser scanning system for scanning a laser beam across a film which is moved by a film transport mechanism. The laser scanning system typically comprises a laser source for generating a laser beam containing input information, a scan lens and a rotating mirror for reflecting the laser beam through the scan lens to produce a scanning beam with a constant linear velocity. The scan lens acts on the scanning beam to provide a focused beam spot that moves in a linear direction across the film, thereby providing a first dimension of the two-dimensional image on the film. Concurrently or alternately with the movement of the scanning beam, the film transport mechanism moves the film either continuously or in discrete steps to provide the other dimension of the desired two-dimensional image.
In order to provide a focused beam spot moving in a linear direction across the film, the laser beam, mirror and scan lens must be precisely aligned with each other. Thus, replacement of any component of the scanning system necessitates re-alignment. Although many different arrangements have been used to scan a laser beam across a material, none so far provide the lack of need for realignment. For example, in the laser scanning system described in U.S. Pat. No. 4,719,474, involving a rotatable prism having two mirror facets, there is still one degree of freedom adjustment which is necessary.
Conventional laser sources for high quality laser scanners, i.e. scanners that produce diffraction-limited beam spots, generally use gas lasers, such as helium-neon lasers. These lasers have low MTBF, require air or gas cooling and cannot be modulated without expensive acousto-optic modulators, which usually have a low band width. Laser diodes, on the other hand, can be readily modulated, but they display very poor beam quality since the beam emitted from a laser diode is elliptical in cross-section and astigmatic, and contains residual optical noise caused by internal reflections, resulting in distortion in temporal and spatial domains. Therefore, laser diodes cannot be used for high quality imaging which requires a laser beam which is circular in cross-section, non-astigmatic and diffraction-limited and has a gaussian energy distribution.